1. Field of the Invention
This invention relates to the shaping and cooling of glass sheets and particularly in the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets.
Shaped glass sheets are widely used as side windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of a glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and into a shaping station where each glass sheet in turn is transferred onto a shaping mold that lifts the glass sheet into engagement with a vacuum mold. The vacuum mold holds the shaped glass by suction while the lifting member retracts to below the substantially horizontal path. The shaped glass sheet is then released from the vacuum mold and conveyed into a cooling station as rapidly as possible where the glass sheet is exposed to cold tempering medium applied at a rate sufficient to impart at least a partial temper in the shaped glass sheet.
When prior art apparatus lifted a shaped glass sheet from a horizontal path of travel into engagement with an upper vacuum shaping mold, it used either a ring-type outline mold having spaces aligned with transversely extending conveyor rolls that conveyed glass sheets into a shaping station, which mold lifted the glass sheet above the path of glass sheet travel or used a shaping mold that was notched along its opposite side edge portions only to provide clearance for the mold to move vertically above the level of the path defined by stub rolls.
Hot glass sheets shaped on an outline mold sag uncontrollably within the outline defined by the outline mold. Hot glass sheets conveyed on stub rolls to a shaping station sag uncontrollably in the space separating the laterally inner ends of opposing stub rolls.
2. The Prior Art
U.S. Pat. No. 3,374,077 to James H. Cypher and U.S. Pat. No. 3,374,080 to Robert W. Wheeler disclose continuous molds disposed horizontally for shaping glass sheets and provided with notched portions at their edges only to provide clearance for movement through a path of travel defined by stub rolls. Stub rolls do not control sagging of glass sheets conveyed therealong in the space between the portions supported by stub rolls, except for relatively thick glass sheets that are not required in present day vehicles where fuel efficiency is important. Also, whenever glass sheets of different transverse dimensions from a previous production pattern must be fabricated, it becomes necessary to adjust the length of the stub rolls as well as replace the mold to produce glass sheets conforming to the new production pattern, regardless of whether the new production pattern has the same or different radius of curvature compared to the previous production pattern. Further adjustment is needed to change for production of glass sheets having different outlines, some of which may be rectangular and others non-rectangular.
U.S. Pat. No. 3,418,098 to William Kirkman and U.S. Pat. No. 3,527,589 to George F. Ritter, Jr., disclose a horizontally disposed shaping mold of the outline type that is attached to and vertically spaced above a supporting plate for bending horizontally oriented glass sheets. The molds are interrupted to provide clearance for the mold to move through a plane occupied by a series of spaced conveyor rolls. Outline molds are incapable of controlling the exact shape to be imparted to the glass sheets within the supported outline. Also, outline molds must be replaced with molds of different outline whenever a different pattern of glass sheets is to be fabricated, even when the radius of curvature to be imparted to glass sheets of different outline configuration is the same. Changing molds for each change of pattern consumes time and any means that reduces the number of mold changes is desirable.
U.S. Pat. No. 3,756,797 to Kazuyuki Akeyoshi et al discloses a mold having grooves extending both longitudinally and transversely of an apertured shaping surface of a glass sheet bending mold of the sag bending type that is moved on rollers 24 through a heating furnace. Heated gas is applied through the apertures and exhausted through the grooves. The grooves are much smaller than the usual size of conveyor rolls used to convey glass sheets into a shaping station.